The definition of cell theory has changed over the years. At first, it meant the specific microstructure of cells. But over many years, several botanists and scientists contributed to and expanded the definition. Today cell theory explains the general principles of construction for all living things. Current cell theory consists of six statements: 1. All living matter is made up of cells. 2. All cells come from preexisting cells (most from cell division, but some from fusion of egg and sperm). 3. A cell is the basic unit of life. 4. Each cell is encased in a plasma membrane (a thin skin that separates it from the environment and other cells). 5. All cells have strong biochemical similarities. 6. Most cells are microscopic in size.

Before the 1800s, the basic units of life were thought to be fibers and vessels--people believed that living organisms just could not be broken down any more...